The valve operating mechanism is used to transfer a motion of the came fixed to the camshaft, which rotates in synchronism with the crankshaft of the engine, to the valve. In order to suppress a friction loss of the relevant portion lower and attain the improvement in fuel consumption performance, in recent years the cam follower unit set forth in Patent Literature 1, for example, is widely used to change the friction in operation from a sliding friction to a rolling friction in this valve operating mechanism. FIG. 1 and FIG. 2 show an example of such cam follower unit. A rocker arm 1 as a main body of the cam follower is supported rotatably in the engine main body (not shown) of the internal combustion engine by a rocker shaft 3 that is inserted into a shaft hole 2 provided in the middle portion in the lengthwise direction (the lateral direction in FIG. 1). Also, an adjust bolt 4 is screwed into a threaded hole formed in a base end portion and is locked by a locknut 5, and thus is fixed to this base end portion (left end portion in FIG. 1) of the rocker arm 1. Also, a base end face (upper end face in FIG. 1) of an engine valve 6 as the suction valve or the exhaust valve, which is supported in the engine main body (not shown) to move reciprocally, is brought into contact with a top end face (lower end face in FIG. 1) of the adjust bolt 4. The engine valve 6 is always energized by a valve spring 7 in the valve closing direction (direction to contact the adjust bolt 4). Therefore, elasticity is given to the rocker arm 1 in the clockwise direction in FIG. 1.
Meanwhile, a roller 8 is fitted rotatably to a top end portion (right end portion in FIG. 1) of the rocker arm 1 via a roller supporting shaft 9, and an outer peripheral face of the roller 8 is brought into contact with an outer peripheral face of a cam 10 by the spring force of the valve spring 7. This cam 10 is formed integrally with a camshaft 11 that rotates together with a crankshaft (not shown), and is supported rotatably in the engine main body. According to this structure, the rotation of the camshaft 11 is converted into the reciprocal swing motion of the rocker arm 1 around the rocker shaft 3, and also the engine valve 6 is moved reciprocally against the spring force of the valve spring 7 or based on the spring force by the rocker arm 1. Then, the opening/closing action of the suction valve or the exhaust valve provided to the cylinder top portion of the engine main body is executed.
In such valve operating mechanism, the portion for supporting rotatably the roller 8 onto the rocker arm 1 via the roller supporting shaft 9 is constructed as shown in FIG. 2. The roller supporting shaft 9 is put across a pair of supporting walls 12, 12 provided in parallel mutually at a distance to the top end portion of the rocker arm 1. Then, the roller 8 is supported rotatably around the center portion, which is positioned between inner surfaces of both supporting walls 12, 12, of the roller supporting shaft 9 via a plurality of needles 13, 13. Both end portions of the roller supporting shaft 9 are fitted into roller shaft holes 14, 14 provided concentrically in the supporting walls 12, 12 respectively, and are supported therein. In this state, outer peripheral portions of the roller supporting shaft 9 on both ends are plastically deformed outward in the radial direction by hitting the top end edge of the caulking jig such as the punch, or the like against both end faces on the outer diameter side. Thus, the roller supporting shaft 9 is coupled/fixed firmly to the roller shaft holes 14, 14.
Also, with respect to the parts that contacts the other member in terms of the rolling contact in use, like the roller incorporated into the cam follower unit as the object of the present invention, the technology concerning the improvement in surface properties of the parts, which contacts the other member in terms of the rolling contact, to improve durability of a flaking life, or the like is set forth in Patent Literature 2, etc., for example. Various technologies are known in the prior art. In the case of the technology set forth in Patent Literature 2 in the prior art, an abrasion whose surface roughness Rmax is 0.3 to 1.5 μm in the random direction is formed on a rolling surface of the rolling element of the bearing, and also a residual stress layer of 500 MPa or more is formed on a surface layer portion. Also, the invention is set forth in Patent Literatures 3 to 5 that a hardness of the surface layer portion is set higher than that of the inside by forming a large number of depressions on the surface by virtue of the barrel finishing, and also a compressive residual stress is generated on the surface layer portion.
The above technologies known in the prior art to improve the durability can achieve the effect in their own way, but there is yet room for improvement from an aspect of ensuring excellent durability at a low cost. In other words, it is preferable from an aspect of suppressing a cost that the surface finish should be executed by not the polishing, or the like, as set forth in Patent Literatures 3 to 5, but the barrel finishing. In this case, when the surface finish is executed by the barrel finishing, the enough durability cannot always be secured under severe conditions unless the properties of minute depressions existing on the surface are proper. That is, it is found by the inventor's study of the present invention that, even when the surface of the cam follower unit is finished simply by the barrel finishing, a sufficiently tough oil film cannot be formed between the faces that contact mutually via a rolling contact.
For example, when the machining conditions in the barrel finishing are not properly chosen to result in a large surface roughness (deep depressions exist on a surface), a part of the oil that is pushed strongly between two faces that contact mutually via a rolling contact escapes from a clearance between the contacting faces to their peripheries, so that a strength of the oil film existing between two faces is lowered. Also, in the cam follower unit for the valve operating mechanism of the engine, a thickness of the oil film that has the large strength to such an extent that it can be prevented that metal contact is generated at the rolling contact portion between outer peripheral surfaces of the roller and the cam is about 1 μm at best. Therefore, when the large projection (which projects largely outward) is present partially, the metal contact takes place between the top portion of this projection and the opponent face, so that an early flaking is easily generated based on a peeling from that portion.
Such problem also arises in other members that are displaced relatively (contact in terms of a rolling contact or a sliding contact) while contacting the surface of the other member at a large face pressure. For example, in the foregoing structure shown in FIGS. 1 and 2, since the cylindrical inner ring raceway portion formed on the outer peripheral surface of the center portion of the roller supporting shaft 9 contacts the rolling surfaces of the needles 13, 13 in terms of a rolling contact at a large contact face pressure, it is possible that satisfactory durability cannot be secured unless the surface properties are proper. Also, in the rocker arm constituting the cam follower unit, the similar problem arises in the structure that the rocker arm sliding-contacts the other member at a high face pressure.
FIGS. 3 and 4 show the sheet metal rocker arm set forth in Patent Literature 6, as an example of a rocker arm 1a in which such problem will arise. The rocker arm 1a is formed by applying the punching and the bending to a sheet of metal plate while using the press. Circular holes 19, 19 into which both end portions of the roller supporting shaft to support the roller 8 are fitted and fixed are formed in the center portion of the rocker arm 1a. A first engaging portion 20 as a partial cylindrical convex face against which the base end face of the engine valve 6 (see FIG. 1) is hit is formed in one end portion (right end portion in FIGS. 3 and 4) of the same. A second engaging portion 21 as a hemispherical concave face against which the top end face of the rush adjuster is hit is formed on the other end of the same.
Such first and second engaging portions 20, 21 are slid minutely over the other face respectively while they come onto contact with the base end face of the engine valve 6 or the top end face of the rush adjuster at a high face pressure in their using condition. Therefore, if the surface properties of the first and second engaging portions 20, 21 are not proper, the metal contact occurs between these engaging portions 20, 21 and the other face when the working conditions are severe, e.g., the lubricating oil supplied to butt faces of these engaging portions 20, 21 and the other face runs short, or the like. Thus, an early flaking is ready to generate based on a peeling from that portion.
Also, as the invention to improve a rolling contact fatigue life of the cylindrical inner ring raceway that exists on the outer peripheral surface of the shaft, such a technology is set forth in Patent Literature 7 that the shaft is made of a steel that contains C of 0.5 to 1.2 wt % and N of 0.05 to 0.4 wt %, the surface layer having a hardness of Hv650 or more and containing a retained austenite of 15 to 40 vol % is formed by the induction hardening process, and a retained austenite in the core portion is set to 0 vol %. In the case of such technology in the prior art, the sufficient durability cannot always be secured when the lubricating conditions are severe. In addition, the technology to form a reaction layer made of phosphate compound consisting of phosphorus and iron on one or both of the inner peripheral surface of the roller constituting the cam follower unit and the outer peripheral surface of the shaft and then stack the process layer, which is obtained by burning a mixture of molybdenum disulfide and poly(tetrafluoroethylene) together with a thermosetting synthetic resin using polyamideimide as the binder, on a surface of this reaction layer is set forth in Patent Literature 8. According to such technology in the prior art, both peripheral surfaces can be protected until the lubricating oil spreads to the contact area of both peripheral surfaces immediately after the running of the engine is started. However, since the process layer is worn out within a relatively short time after the running of the engine is started, such process layer is seldom helpful in protecting the surface of the rolling sliding parts in the lubricating oil insufficient state that is generated after a certain time has elapsed from the start of the running.
Patent Literature 1: Japanese utility model publication Bo. Sho. 60-88016
Patent Literature 2: Publication of Japanese translation of International application No. Hei. 1-30008
Patent Literature 3: Japanese patent unexamined publication No. Hei. 3-117723
Patent Literature 4: Japanese patent unexamined publication No. Hei. 3-117724
Patent Literature 5: Japanese patent unexamined publication No. Hei. 3-117725
Patent Literature 6: Japanese patent unexamined publication No. 2001-280106
Patent Literature 7: Japanese patent unexamined publication No. 2002-4003
Patent Literature 8: Japanese Patent No. 3496286